1. Field of the Invention
The present invention relates to a method of producing a field effect transistor having a carrier moving layer formed by group III nitride semiconductor material, GaAs or SiC.
2. Description of the Related Art
Recently, semiconductor devices using the group III nitride semiconductor material have been developed to increase in operational speed and to secure stable operation under high temperature condition (see, for example, Japanese patent application publication No. 2000-174034).
As such group III nitride semiconductor, there are exemplified GaN, InGaN, for example. When a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is fabricated using the group III nitride semiconductor, a high temperature annealing treatment is applied in an electric furnace or a RTA (Rapid Thermal Anneal) equipment. This treatment causes impurities implanted in a source forming region or a drain forming region in the semiconductor layer to function as a dopant, thus activating the impurities (see Y. Irokawa et al., “Implantation temperature dependence of Si activation in AlGaN,” Appl. Phys. Lett. 88 (2006) 182106. S Matsunaga et al., “Silicon implantation in epitaxial GaN layers: Encapsultant annealing and electrical properties,” J. Appl. Phys. 95 (2004) 2461. J. A. Fellows, “Electrical activities studies of GaN implanted with Si from low to high dose,” Appl. Phys. Lett. 80 (2002) 1930).
Incidentally, in the heat treatment such as the high temperature annealing, an infrared light is irradiated to the substrate or a sample stage or the like. The irradiated light heats the substrate or a sample stage so that the temperature of the semiconductor layer is raised by the heat transfer so as to activate the impurities implanted in the semiconductor layer.
Very high temperature of the substrate is required in the fabricating method in which the impurities implanted in the semiconductor layer are activated by the thermal conductivity from the substrate or the sample stage. For example, when a GaN film is used as the semiconductor layer, at least 1100 degrees centigrade of temperature is required as the sample stage temperature so that the temperature of the electric furnace is set to be at least 1200 degrees centigrade to attain the above described temperature of the sample stage.
Since the above described temperature is over the temperature at which the GaN film grows, the Ga droplets on the surface of the GaN film, or the nitrogen atom desorbs the GaN film and then the Ga atom evaporated the GaN film. Then, further, the crystalline defect called as a pit is generated on the surface of the GaN film depending on a cycle in which the nitrogen atom desorbs. For example, when a sapphire substrate is used, the Ga desorbs or the pit is formed on the surface of the GaN film after being heated for 10 seconds in the RTA at the temperature of at least 1200 degrees centigrade. In addition, when the silicon (Si) substrate is used, the Ga droplets or the pit is formed on the surface of the GaN film after being heated for 10 seconds in the RTA at the temperature of at least 1100 degrees centigrade. Furthermore, the GaN film is peeled off the Si substrate, or the Gan film is broken after being heated at least 10 seconds in the electric furnace at the temperature of at least 1200 degrees centigrade. This phenomenon is due to a large difference in the heat expansion coefficient, the lattice constant, and the heat transfer rate between the Si substrate and the GaN film, thus not enabling to fabricate the FET.
Furthermore, among the above described problems, the segregation of the Ga atom forming the semiconductor layer and the formation of the pit appears even in a region where the impurities are not implanted, for example the region directly underneath the gate insulation film. Accordingly, when a MOSFET is fabricated applying the above described conventional techniques, the channel mobility movement is lowered in the channel region, or blocking characteristic thereof is deteriorated. In addition, when a HEMT (High Electron Mobility Transistor) is fabricated, the density of the two-dimensional electron gas is reduced.
One of the objects of the invention is to provide a method of producing a field effect transistor which enables to activate the implanted impurities without deteriorating the performance and reliability of the device.